Posts Tagged ‘Green Stormwater Infrastructure’

A Watershed Approach to Green Infrastructure

“Water, water everywhere, nor any drop to drink!” Do you remember this from The Rime of the Ancient Mariner, a poem by Samuel Taylor Coleridge? In the story, the Mariner is lost at sea with a dead albatross around his neck, and although surrounded by water, he is dying of thirst because the ocean water is undrinkable. At the end of the story, the Mariner awakes the next morning “a sadder and a wiser man.”

Today in America we face a similar situation. There are over 42,000 impaired waterways in the United States. An ‘impaired waterway’ is a lake, river, stream or estuary that is too polluted to meet water quality standards. An ‘impaired waterway’ is the nice way of saying the water is dangerous to wildlife and human health. The U.S. Environmental Protection Agency (EPA) estimates that as much as 40% of our nation’s lakes, rivers, and streams are not safe for swimming, fishing, or drinking. Incredibly in some states, over 80% of the waterways are not safe for these activities.

Over the last forty-five years we have come a long way in improving water quality. In the 1960’s Johnny Carson joked that he took a walk on the Hudson River. The Hudson River was so polluted you could almost walk on it. In 1968 the Cuyahoga River in Northeast Ohio caught fire for the last time. Since 1868, the Cuyahoga River, made famous by being the “river that caught fire,” actually caught fire 13 times. These two impaired waterways helped propel the environmental movement. In 1972, the Clean Water Act was passed into law and the task of cleaning up our polluted waterways began. The purpose of the Clean Water Act is to restore and maintain the quality of our nation’s waters by preventing point and nonpoint source pollution. Overall, we have done a great job of fixing point source pollution, or single identifiable source, problems.

Nonpoint Source Pollution

Toxic algae bloom from stormwater runoff

If this is the case, why are over 40% of our lakes, rivers, and streams still unsafe for swimming, fishing, or drinking? The main culprit is nonpoint source pollution. Nonpoint source pollution is the greatest threat to water quality in our nation, i.e. stormwater. When it rains, the stormwater is rapidly collected, piped, and swiftly dumped into the closest waterway. Stormwater, as it travels across the surface of the land, carries with it all the pollutants from the landscape. In agricultural landscapes, excess nitrogen, phosphorus, and pesticides are concentrated in the nearest body of water.

In urban areas, in addition to lawn chemicals, all the oils, grease, salts, and heavy metals from our roadways are deposited in our local waterways. In many urban areas, our stormwater systems are combined with our sewerage systems. When a stormwater surge occurs, the sewage system is not large enough to handle the volume of water from the rainfall. Rainwater mixed with sewer water overflows, untreated, into the nearest local waterway. In many cites, a sewage overflow may occur with as little as one-fourth inch of rain. Take for example Indianapolis, Indiana, where the city experiences 50 to 60 overflow events every year.

In the past, civil engineers would say, “The solution to pollution is dilution.” This meaning, if you had enough clean water running through the system, a little bit of pollution would not be noticeable. There is a lot of evidence that this assumption is grossly inaccurate.

We have reached our limit in diluting the pollution we create by dumping our wastewater into our local waterways. Today, the “dead zone” in the Gulf of Mexico is the size of Connecticut. The dead zone is an area in the Gulf of Mexico almost completely devoid of any life because of a lack of oxygen in the water. The dead zone is a direct result of fertilizer runoff from the rivers and streams upstream from the Gulf. The excessive fertilizer runoff results in algae blooms. When the algae die, the process of decomposition consumes oxygen, creating the oxygen depleted “dead zone” in the Gulf of Mexico. With advancements in green infrastructure, we have an opportunity to clean up our waterways and enhance our environment.

The U.S. Environmental Protection Agency (EPA) has developed extensive research demonstrating that green or more natural solutions to stormwater runoff are less expensive than conventional grey stormwater solutions. Additionally, green stormwater solutions create an array of broader benefits for local economies and the environment. From an environmental standpoint, instead of trying to minimize the impact we have on the environment we can actually enhance our environment through the utilization of living plants when creating green infrastructure. We have an opportunity to make our environment better for our children and their children.

Green infrastructure is a natural approach to stormwater mitigation that brings nature back into the fold. Before urban and agricultural development, in a deciduous hardwood forest or native prairie, less than 1% of rainfall ran off the surface of the land into the local streams. In the natural hydrologic cycle, the continuous movement of water above and below the earth’s surface, 10% to 40% of rainfall would go into the ground and recharge our aquifers, 40% to 50% went back into our atmosphere as evapotranspiration, and 20% to 30% would go into an interflow layer of soil. The interflow layer of the soil, when undisturbed by human development, contains organic matter, microbes, and plant roots that would extend downward twelve to thirty feet or more. The plant roots and organic matter helped to maintain the porosity and permeability of the soil. This is the primary reason only 1% of rain would run off the surface of the native landscape directly into the nearest body of water. This interflow layer or topsoil layer of a native landscape acts like an old-fashioned wastewater treatment plant or a biologically balanced aquarium. As the stormwater drains into the interflow layer, the microbes in the soil clean the water of harmful pollutants. Eventually the interflow water travels downstream into the nearest waterway. The stormwater enters the local waterway cleanly and slowly.

Stream Bank Erosion

Non-point source pollution from agricultural runoff.

Another problem with our current grey infrastructure approach to stormwater management is stream bank erosion. To prevent flooding, we rapidly collect rainwater, pipe it, and deposit it in the nearest body of water. The rapid collection of stormwater increases the velocity and volume of water in our local waterways.

The excessive flow of stormwater into our streams scours the stream banks increasing stream bank erosion. Stream bank erosion increases the sediment that streams carry, resulting in the loss of fertile bottomland, and a decrease in habitat for species on land and in the stream.

If you find yourself in a forest during a rain, you will notice immediately that you hardly get wet. The leaves of the trees slow the rain falling from the sky, slowly dropping the water onto the floor of the forest. When you walk through a native forest, you will also notice the forest floor feels spongy and soft. In an undisturbed native forest or prairie, the plant roots and soil organic matter maintain a soil structure that is loose, friable, and capable of absorbing a lot of rainwater. In contrast, today our urban landscapes runoff over 90% of the rainwater that falls from the sky; suburban landscapes rainfall runoff is over 60%; and even agricultural areas will runoff over 40% of the rainfall.

Over the last twenty years our impermeable surface area has increased by over 40%, mainly because of parking regulations and wider streets. Current site development techniques strip off almost all the topsoil and heavy construction equipment compacts the remaining subsoil to over 90%. When soils are compacted by mechanical forces the soil structured is destroyed. The result is reduced soil porosity that limits water infiltration. A soil that is 90% compacted has the consistency, density, and firmness of a gravel road.

These practices highly reduce rainwater absorption rates, even within intensively landscaped areas. With one-inch of rainfall, a 2,500 square foot roof will generate 1,500 gallons of water. On a one-quarter acre residential property, a one-inch rainfall will deliver almost 7,000 gallons of water. A city block of five acres will experience 135,000 gallons of water, or the equivalent of more than five average-sized swimming pools.

When you consider a watershed, the volume of water from a one-inch rainfall can easily exceed several million gallons of water. Unlike today, in the past most of this rainwater stayed on-site, with very little runoff into the local streams and waterways. As you start to understand the volume of water from stormwater and the impairments that have occurred due to our current land use practices, it becomes clear why the EPA has designated stormwater as the largest source of water pollution in America. In the recently released five-year strategic plan, the EPA designates protecting America’s water as second in priority, only surpassed by addressing climate change and improving air quality.

Greening Our Grey Infrastructure

A bioswale creates a more natural approach to stormwater mitigation.

So where does green infrastructure fit in? The three primary concepts in developing a green infrastructure approach to stormwater are:

Capture rainwater as close to the source as possible;

Slow down rainwater flow rate; and

Filter rainwater through absorption. In natural forest and native prairie areas rain is soaked into the land, filtered through the soil, and applied back into the landscape.

There are many tools used in developing green infrastructure. Rain gardens, bioswales, and infiltration planters use plant material to retain and filter rainwater. Permeable paving materials help to reduce the volume and velocity of stormwater while filtering out heavy metals, grease, and oils. Reconstructed wetlands, like naturally occurring wetlands, act like kidneys of the ecosystem. And planting trees and native grasses along stream banks can be more effective and less expensive than conventional stream armoring techniques of rock riprap and gabion cages.

Green infrastructure works best when it is combined with a comprehensive stormwater management plan based upon the existing watershed. Combining green infrastructure with existing grey infrastructure is the most cost effective solution to solving our nonpoint source pollution problems. American Rivers published a study three years ago, entitled The Value of Green Infrastructure, that found green infrastructure, when properly deployed, created more jobs for the longest period and for the least amount of money. Public education is crucial to the successful development and implementation of a comprehensive green infrastructure plan.

Rain garden under construction

Some of the best success stories come out of Portland, Seattle, Chicago, and Philadelphia where communities are working together taking a neighborhood approach to stormwater mitigation. The conventional approach to stormwater issues is to fix the problem where the problem occurs, at the point of convergence. We do not pay attention to the source of the problem, usually further upstream. We install a storm drain and connect it to the nearest stormwater or sewage pipe. After fifty years of taking this approach we find ourselves constructing sewage treatment systems and conveyance systems five times larger than needed so the current system can handle the stormwater with our antiqued sewerage treatment plants. Handling stormwater like sewer water is not only wasteful but it is extremely expensive.

If we take a watershed approach utilizing the tools of green infrastructure, stormwater is treated as close to the source as possible. The root problem is addressed on-site without expensive conveyance systems and oversized sewage treatment facilities. Handling stormwater as close to the source as possible requires a green infrastructure design solution as far upstream in the watershed as possible. If done correctly, this method has proven to be the most effective and usually least expensive means of mitigating stormwater.

Green Infrastructure in Practice

Stormwater flow map

In a 50-year-old condominium complex, built next to a major creek, a huge stormwater issue was created that eroded the stream banks and threatened the structural foundation of several condominium units. Several years ago, an expensive and conventional gabion basket retaining wall was installed to stabilize the stream banks against further erosion. It was quickly discovered that this grey infrastructure approach only moved the problem further down stream.

After installing a test rain garden to reduce a flooding basement issue, the homeowners association was convinced of the effectiveness of green infrastructure. Subsequently, a green infrastructure plan was developed based upon stormwater received by each of four drainage basins or watersheds on their 78-acre parcel of land. It was discovered that most of the stormwater causing the problem was coming upstream past the property line. Working together with surrounding community neighbors proved to be the most cost effective way to deal with what could be a very expensive stormwater issue.

Stormwater issue map

The completed green infrastructure plan developed for this condominium complex clearly identified how much stormwater volume needed to be handled. The perimeter of the property will be phase one of a ten-year plan. Subsequent phases of the plan, which are downstream from the initial phases, will be evaluated to determine the effectiveness of the green infrastructure solutions implemented in previous years. What was discovered during the development of this plan was that the further upstream we go to develop a stormwater solution the less expensive the solution is and the more effective it is to handle the larger volumes that converge downstream.

Planning Our Consequences

Ideally, the long-term goal of any green infrastructure plan is that no piece of property within a watershed or stormwater basin has any stormwater runoff. If zero stormwater runoff can be achieved, land use planning becomes a dynamic tool that addresses the largest source of pollution in America today. Developing a watershed plan utilizing green infrastructure design takes a completely unconventional approach to stormwater mitigation.

We are learning that green infrastructure mimics the natural water cycle. Green infrastructure develops better filtration, filtration that we do not have in our conventional approach. Moreover, green infrastructure is often significantly less expensive than conventional grey infrastructure.

Natural water cycle

Grey infrastructure, although expensive, does an excellent job at reducing flooding. However, grey infrastructure fails miserably in reducing stormwater runoff and reducing pollution. Grey infrastructure works well for the 100-year rainfall events but does little for the frequent one-inch rainfall events. This is where green infrastructure shines. For the one- to three-inch rainfall events, green infrastructure works best. Combining green infrastructure with existing grey infrastructure utilizes the best of new technology with conventional technology, creating a system that is not only more effective by also more economical. This is the next step in an evolutionary approach to better understanding our environment and the future.

This new paradigm of understanding nature applies to stormwater mitigation, and many other environmental issues we currently face. As these new concepts are considered it is important to remember: in nature there is no good, there is no bad, there are only consequences.